Sound reproducing system



Feb. 18, 1936. B. oLNl-:Y 2,031,500

SOUND .REPRODUCING SYSTEM Filed Sept. 1'7, 1934 2 Sheets-Sheet l ATTOR NEY Feb. 18, 1936. B. OLNEY SOUND REPRODUCING SYSTEM 2 sheets-sheet 2Filed Spt. 17, 1954 mwN mami

INVENTOR BENJAWHN OLNEY. BY Wm ATTORNEY Patented Feb. 18,1936

`UNITED STATES PATENT OFFICE l manson soUNn nEPnooUcmG SYSTEM New YorkApplication September 17, 1934, Serial No. 744.382

1i) Claims.

'Ihis invention relates to sound reproducing systems.

When a loud speaker of the open radiator type such as a cone type loudspeaker is mounted in a cabinet, the sound reproduction thereof isalways marred by so-called cavity resonance which manifests itself in adisagreeable boominess and an apparent lowering of the pitch of speakingvoices. This is due to the fact that a loud speaker cabinet, althoughopen at a side or an end, otherwise encloses a column of air at the rearof the loud speakerand therefore acts like any wellknown resonator inwhich the resonance is due to the reaction between the mass andthestiffness of a partially enclosed column of air. The resonant frequencyand the intensity of resonance of such a cabinet varies with itsposition relative to the walls of the room and surrounding objects, theresonant frequency Vbeing lowered and the strength of the resonancebeing augmented when the back of the cabinet is -placed close against awall. vided with an open radiator type of loud speaker may be minimizedby careful cabinet design, in

so far as applicant is aware, such resonance has never been completelyeliminated. Y It is recognized that a. cone type loud speaker mounted ina baille of limited size is very inefficient in handling power at lowfrequencies due to the fact that the loud speaker has an insufficientacoustic load at certain of the lower frequencies and consequently, atthese lower frequencies, the loud speaker may chatter as a resuit of itsmoving coil striking the pole pieces of the speaker on relatively largesurges of current in the moving coil such as occur when loud passages ofmusic or the like are reproduced.

Commercial open radiator or cone types of loud speaker have a poorresponse at the lower frequencies, notably frequencies below one hundredcycles, so thatbass voices and the music from bass instruments are onlyweakly reproduced thereby at such frequencies.

In accordance with the main feature of the present invention, means areprovided in a sound reproducing system which substantially eliminatesthe air cavity resonance effect of a loud speaker cabinet .forming apart of such system.

An additional feature of the invention relates to means in a soundreproducingsystem for suppressing or absorbing substantially. all of thesound waves of higher frequencies transmitted -from one surface of anexposed radiator such-as a cone type loud speaker incorporated'in saidsystem. while discharging into free air, certain un- While cavityresonance inV a cabinet proabsorbed low frequencies transmitted `fromsaid surface.

A further feature of the invention relates to a loud speaker systemprovided with a conduit or passageway lined with sound absorbingmaterial and having substantially the form of a labyrinth into which thesound Waves radiated from one surface of a sound radiator of the loudspeaker,

are discharged.

Still a further feature of the invention relates to a loud speakersystem provided with a conduit suitably lined with sound absorbingmaterialand of such length as to resonate at those lower frequencies atwhich the loud speaker is an inefficient reproducer so that' the lowfrequency response of the loud speaker is. extended.

These and other features of the invention will appear from the detaileddescription and claims when taken with the drawings in which Fig.- 1 isa fragmentary plan view of a cabinet containing the sound reproducingarrangement of the present invention; Fig. 2 is a vertical sectionsection thereof taken substantially on the line 2'-2 of Fig. 3; Fig. 3is a vertical sectional view taken substantially on the line 3-3 of Fig.2; Fig. 4 is a chart showing the differences in response of a givensound reproducing system with and Without the arrangement disclosed inthe foregoing figures; Fig. 5 is a fragmentary plan view of a cabinetcontaining a modified type of sound reproducing system; Fig. 6 is avertical section view thereof taken on the line 6 6 of Fig."7; andFig.'l is a sectional view taken substantially on the line'l-l of Fig. 6.While the invention may have various embodiments, one form thereof isillustrated in Figs. 1, 2 `and 3 in which the sound reproducing systemis housed in a cabinet made of wood or like materiPl. This cabinetincludes a top 5, sides 6 and l, and a front or baille 8, vthe rear andbottom of thisv cabinet being conveniently left open. A loud speaker ofthe open radiator type such as a dynamic cone type loud speaker El ismounted in the cabinet with the frame ill (Fig.` 3) of its diaphragm Ilclosely engaging the margin of an opening i2 in the baille 8 so that thefront surface of the diaphragm transmits reproduced sound waves throughthis opening. It will be understood that. in the construction justdescribed, which follows current practice, the rear surface of thediaphragm transmits sound waves'into the space within the cabinet. Thetop, sides and front Such a sound reproducing system is also deficientin thatit affords poor reproduction ofthe low frequencies notably thosebelow. one hundred cycles as indicated at D in the response curve A ofFig. 4. This poor reproduction at such ylovv frequencies arises from thefact that it is imprac- -tical to provide a commercial cabinet having.ai

suiiiciently large baille area to prevent the circulation of airfrom thefront surface ofA the diaphragm to the rear surface'thereof and viceversa. This poor'reproduction at the mentioned low frequencies alsoarises from the fact that prior loud speakers having an open Atype ofradiator, could not be provided with a sufiicient acoustic load toprevent the moving coil of the diaphragm from striking the pole piecesof the actuating unit of the loud speaker on relatively large surges ofcurrent in the moving coil, such as occur in the reproduction of loudpassages of music and the like. If the moving coil were permitted tostrike the pole' pieces in the manner just described, itwould cause arattling or chattering sound which would be highly unpleasant to thelistener. In -order to prevent such rattling of the loud speaker, ithasbeen the common pracitice-to provide such a rigid mounting for the`moving coil thereof that this coil could notstrike the pole pieces ofthe actuating unit. However, such a rigid' mounting prevented thediaphragm vfrommoving freely in response to the currents'of lowerfrequency and consequently vthe sound reproducing system had a. poor lowfrequency response.

In accordance with the present invention an arrangement is providedwhich' eliminates the disadvantages of such former sound reproducingsystems, While this arrangement maytake various forms', as shown inFigs. 1, 2 and 3,it com'- prises a housing made in two parts designatedI 3. I3 which with the bracket III" forming a .part of the frame ofthe'lo'ud speaker completely encloses.

the rear surface of the diaphragm I I so that none of the sound waves'from the rear surface of this diaphragm are transmitted into the spacewithin the cabinet. It will be noted that .the interior of /the housingcomprising the parts I3, I3, is lined with felt Il or other suitablesound absorbing material. The parts I2 and I3 respectively communicatewith the conduit I1 through theslots or openings I5 land I6 which aresymmetrically placed with respect to therear surface of the diaphragm IISubstantially all of the inner sur- Y faces of this conduit are linedwith a sound absorbing material I8 such as rock wool or other -materialof like characteristics. Since it is desir- 'able to have the conduit I1of relatively great length and since it is highly desirable incommercial cabinets that they be of relatively small sue. the conduit isherein shown as being folded back and forth upon itself, in other words,it com'- prises a circuitous passageway. The conduit I1 discharges intofree air through a slot t9 at the front surface of the diaphragm to .therear surface thereof and vice versa.

It should bementioned `,that the conduit is preferably made of uniformrectangular cross section since by this arrangement a maximum area ofsound absorbing material can be brought in contact with sound waves inany given cabinet.

the air in the conduit may not become suflicientr 1y great to. interferewith the motion of the loud speaker diaphragm at low frequencies. Thiscross sectional area is also made large because of the fact that in apipe of large proportions,

the overtones are thrown somewhat out'of tune with the' pipe and are notreadily excited therein. The cross section of the con`duit andconsequently, the shape of its orii'lce is in a form of an elongatedsl`ot which provides poor efciency for the radiation of low frequencysounds. 'This reduces the radiation which occurs at 'the fundamentalresonance of the conduit. As a result of' the aeoustically remotepositioning of the orifice I9 with respect to the front surface of thediaphragm, the loss of lowfrequenciesdue to circulation between the twosurfaces of the diaphragm is largely prevented. While there/can bereasonable variations in the cross sectional area of the conduit and itslength, it has been found empirically that a conduit having -across-section of practice by the amount of room available in thecabinet: with some sacrifice in results, but the ,ideal length however,is suchthat the fundamental resonanceof the pipe occurs just below thefrequency at whichthe loud speaker cabinet be- .cmes increasinglyineffective as a baille as diagrammatically indicated by the portion Dof the response curve A of Fig. 4. By 'proper proportioning ofthe'dimensions of the conduit, this reson- `ance may be caused toextend, in a smooth manner, the response of the loud speaker at thelower frequencies where ordinarily the efilciency would be poor due tolimitations in bale size. 'I'he sound absorbing lining of theA conduittogether with the' poor radiation afforded by the slot shaped orifice I9prevent the fundamental resonance from being excessively strong and alsoserve to broadenA this resonance considerably. As aresult of adding thepresent invention to the cab- `inet previously referred to, the responseof the sound reproducing system is transformed from its condition asindicated in the broken line response curve' of Fig. 4 tothe conditionindicated by the response curve B illustrated in full lines in thisfigure. From this chart it will. be seen that the low frequency responseof the system has been greatly extended in rangeand the resonant orpeaked frequencies have beeny largely reduced so that the systemaffords/a uniform o'r substantially straight line response over anextended frequency range. v

It has been mentioned that the openings IB and I6 in the housingaresymmetrically placed with respect to the rear surface of the diaphragm.

This is done so that the diaphragm may yniet be driven out of line dueto an unbalanced acoustic load.

It should also be mentioned that experiments thus far conducted indicatethat the acoustical impedance presented to the rear of the diaphragm bythe conduit I1, at very low audible frequencies has the characteristicsof a mass reactance which is large as compared with that of saiddiaphragm including the acoustic load on its freely transmitting side,so that it is possible to greatly reduce the rigidity of the centermount ing of the diaphragm Il without its moving coil lstriking the polepieces on relatively large surges of current. This reduced rigidity ofthe mounting permits the diaphragm to respond more freely at thementioned lower frequencies. This acoustic impedance at one low audiblefrequency lined with felt 32.

has the characteristics of a pure resistance while at audiblefrequencies higher than the last-mentioned frequency, this impedance hasthe characteristics of a compliance reactance associated with aresistance, the magnitude of the resistance component with respect tothe compliance component becoming greater as the frequency increases andbecoming predominant at the higher audible frequencies. Thesecharacteristics of the mentioned acoustic impedance together with theresonance of the conduit l1 at the lower frequencies explain the greatlyimproved lower frequency response of the present sound reproducingsystem.

In certain sound reproducing systems where it is desirable to providealoud speaker having a diaphragm of `greater diameter than thatillustrated in Fig.v 1, it may not be convenient to arrange the conduitin the manner shown in Fig. 3. Under such conditions, a modified form ofthe invention as shown in Figs. 5, 6 and 7 may be employed. As beforethe system is housed in a cabinet having a top 2l, sides 26 and 21,' andbaffle of well-known construction which cabinet can conveniently be openat the rear and the bottom. In this arrangement the loud speaker whichincludes the diaphragm or radiator 28, is suitably mounted behindtheir-ont` or baille of the cabinet with the front surfaee'jgf thediaphragm transmitting sound waves generated thereby through thediaphragm 28 is enclosed'in a housing generally designated 3iwhich-housing is suitably This housing has openings 33 and 33respectively communicating with two separate conduits 34 and 35. Each ofthese conduits is lined with a sound absorbing material 38 such as rockwool or the like and is folded back and forth upon itself as acircuitous passageway in. order to afford relatively great length' forthe conduit while still maintaining it in compact form. These conduitsrespectively open into free air through rectangular slots 36 and 31 inthe bottom o f the cabinet. This arrangement of these mentioned slotseffects the discharge of the sound waves from the rear surface of thediaphragm as acoustically remote as'possible from the frontsurfaceof`the diaphragm as is consistent with a given cabinet size.these conduits preferably hasits dimensions comparable with those of theconduit of Figs. 1, 2 and 3.

Listening tests of the above systems show an entire lack of thecustomary boominess associated with cabinet loud speakers and it hasbeen found that the naturalness of the reproduction has been greatlyincreased.l

The rear surface of Each 'of In addition to the improvement inreproduction described above, there are other advantages to be had fromthe use of the system, of the present invention. The loud speakercabinet may be placed tightly against the wall without changing itsoperating characteristics or it may be used, for example, on a stagewhere the acoustics at the back of the loud speaker may be poor andwhich would give inferior results with the loud speaker radiating freelyfrom both sides of its diaphragm. 0n account of the loud speakerradiating from one side only of its diaphragm over the greater part ofthe frequency range, the standing wave pattern in the room due to theinterference of sound waves is greatly simplified. Furthermore, sincesound is not being discharged into the back of the cabinet, thelikelihood of acoustic coupling due to the mechanical vibration of thevacuum tubes and condenser plates in the radio receiver chassis which iscommonly mounted within the cabinet, is greatly reduced. For a likereason there is much less trouble from rattling of the cabinet and theradio receiver chassis parts, due to acoustic driving.

What I claim is:

1. The method of improving the sound reproduction of an open diaphragmtype of loud speaker which comprises transmitting to the atmospheresubstantially unmodified sound Waves from one surface of said diaphragm,absorbing in a folded passageway by `multiple reflection from thesurface of material having a high degree of acoustic absorption, themajor portion of 'the range of audio frequencies of sound wavestransmitted from the other surface of said diaphragm, and simultaneouslyresonating a certain remaining portion of the frequency range of soundwaves transmitted from said last-mentioned diaphragm surface'.

2. The method of improving the sound reproduction of an open diaphragmtype of loud speaker which comprises transmitting substantiallyunmodified sound waves to the atmosphere from one surface of saiddiaphragm, absorbing substantially al1 frequencies above 100 cycles`transmitted from the other surface of said diaphragm, resonatingcerta-in of the unabsorbed frequencies radiated from the last-mentionedsurface of the diaphragm, discharging the resonated frequencies at apoint remote from the first-mentioned surface of said diaphragm. andutilizing the resonated frequencies to augment the response of the loudspeaker at certain frequencies.

3. In vcombination with a loud speaker having an exposed radiatingdiaphragm and actuating means therefor, a housing enclosing one surfaceonly of said diaphragm, the other surface of said diaphragm being infree and unrestricted communication with the atmosphere, and a sounddamping conduit communicating with the interior of said housing andextending therefrom to free air at a, point remote from the othersurface of said diaphragm, said conduit comprising naterial having ahigh degree of acoustic absorpion.

4. In combination with a loud speaker having an exposed radiatingdiaphragm and actuating means therefor, a, housing enclosing one surfaceonly of said diaphragm the other surface of said diaphragm being in freeand unrestricted communication with the atmosphere and a folded soundabsorbing conduit of substantial length opening in one wall thereof, aloud speaker having an exposed radiating diaphragm one surface of whichis arranged to transmit sound unrestrictedly through said opening to theatmosphere and actuating means for said diaphragm, alhousing enclosingthe other surface of said diaphragm and a sound damping structure withinthe cabinet defining a circuitous sound passageway extending from theinterior of said housing to free air at a pointflocated as accusticallyremote as possible from Athe first mentioned surface of the diaphragmconsistent with the size of the cabinet.

6. In combination with a loud speaker having an open type of radiatingdiaphragm and actuating means therefor, a housing enclosing one surfaceonly of said diaphragm the other surface of said diaphragm being in freeand unrestricted communication with the atmosphere, and a circuitoussound damping conduit communicating with the interior of said housingand extending therefrom to free air, the cross sectional area of saidconduit being at least equal to one-half of the projected area of saiddiaphragm.

7. In combination with a loud speaker having an open radiating diaphragmand actuating means therefor, a housing enclosing one surface only ofsaid diaphragm. the other surface of said diaphragm being in 'free andunrestricted communication with the atmosphere, and a circuity ous sounddamping conduit lined with sound absorbing material communicating withthe interi-or of said housing and extending therefrom to free air.

8. In combination with aloud speaker having an open radiating diaphragmand actuating means therefore, a housing enclosing one surface only ofsaid diaphragm, the other surface of said Adiaphragm being in free andunrestricted communication with the `Iatmosphere, and a pair of sounddamping condits opening into said housing symmetrically with respect tosaid surface of said diaphragm and communicating with freeair,

said conduits having their inner surfaces treatedl point outside of saidcabinetsaidlconduit in-y cluding sound damping material and having alength greater than the greatest dimension of said cabinet whereby soundradiated from the other surface ofy said' radiator is not dischargedwithin the enclosure of said cabinet. g

10. In-a sound reproducing system, a cabinet having anopening in one walthereof, a. loud speaker mounted in said cabinet and including aradiator the front surface of which radiates sound only through saidopening in free and unrestricted communication with the atm'osphere, anda folded conduit for resonating and conducting certain of thefrequencies of the sound radiated from therear surface of said radiatorto apoint acoustically remote from saidffront surface.

11. In avsound reproducing system, a cabinet having an opening in onewall thereof, a loud speaker mounted in said cabinet and including aradiator of the type which radiates sounds from both surfaces thereof.the front surface of said radiator radiating -unmodified sound onlythrough said opening to the atmosphere, and a folded conduit forconducting sound radiated from the rear surface of said radiator toapoint acoustically remote from said surface, said conduit being linedwith acoustically absorbent material.4

12. In a sound reproducing system, a cabinet having an opening in onewall thereof, a loud speaker mounted 4in said cabinet and including .adiaphragm of the type which radiates sounds from both of its surfaces,the front surface of said diaphragm radiating unmodified sound to `theatmosphere only through said opening, and aconduit of uniform crosssection throughout its length for conducting sound radiated from therear surface of the diaphragm to a point acoustically remote from thefront surface thereof, said conduit being lined with acousticallyabsorbing material.

13. In a sound reproducing system, a cabinet having an opening in onewall` thereof, aloud speaker mounted in said cabinet and including adiaphragm of. the type which radiates sounds from both of its surfaces,one of said surfaces radiating unmodified sound to the atmosphere onlythroigh said opening, and a conduit of uniform cross section throughoutits length for couducting sound radiated from the other surface of thediaphragm to a point acoustically remote from said opening, said conduithaving a cross section of which the ratio of length -to width thereof isgreater than 1 to 3.7.

14,. Ina sound reproducing system, a cabinet having an opening in onewall thereof, a loud speaker mounted in said cabinet and including adiaphragm of the type which radiates sounds of effective conduit lengthin inches to cross sectional area in square inches less than 1 to 3L5.

15. In a sound reproducing system, a cabinet having an opening in onewall thereof, a loud speaker mounted in said cabinetv and including,

said diaphragm through said barile opening, ac-

tuating means for said diaphragm, and means including a conduit treatedinternally with sound vabsorbing material and leading from the othersurface of said diaphragm to free air at a point remote from saidfirst-mentioned surface of said diaphragm, said conduit having arelatively s'mall cross-sectional area as compared with the area of saidbaille.

17. In combination, a baille having an opening therein, a loud speakerincluding an exposed radiating diaphragm having one of its surfacesladjacent said openingfor transmitting substanradiate unresonated soundto the atmosphere from said surface only through said opening, and meansincluding a folded conduit having its inner surface treated with soundabsorbing material and leading from the other surface of said diaphragmto free air at a point acoustically `remote from said first-mentionedsurface.

19. In combination, a baille havingan opening therein, a loud speakerincluding an exposed radiating diaphragm having one of its surfaces`adjacent said opening transmitting unresonated sound therethrough tothe atmosphere, actuating means for said loud speaker, a housingenclosing one surface only of said diaphragm and a. folded soundabsorbing conduit of substantial length leading from the interior ofsaid housing 'to an opening discharging into substantial free air, thegeneral plane of the margin of said discharge opening extending in adirection substantially at right angles to the principal plane 15 ofsaid baffle.

BENJAMIN OLNEY,

